Prevention of the driving wheels from an excessive slip or spinning on a road surface is extremely useful for the sake of safety as well as for an effective provision with the driving force of a vehicle. The prevention may be achieved by decreasing a torque to be transmitted to the driving wheels--a torque being the cause of the slip or spinning.
Slip control systems of this type are disclosed in Japanese Patent Early Publication (Laid-Open) No. 16,948/1983 (corresponding to U.S. Pat. No 4,484,280) and Japanese Patent Early Publication (Laid-Open) No. 56,662/1985 (corresponding to U.S. Pat. No. 4,583,611). The systems disclosed in these two prior applications involve, in each case, a technique using the application of a braking force by the engine to the driving wheels and a reduction in the output torque of the engine itself in order to decrease the torque to be transmitted to the driving wheels. More specifically, Japanese Patent Early Publication No. 16,948/1983 discloses a system in which the braking of the driving wheels only is carried out when a slip of the driving wheels is small, on the one hand, and the output torque of the engine is caused to be decreased, in addition to the braking of the driving wheels, when the slip of the driving wheels becomes large, on the other hand. Japanese Patent Early Publication No. 56,662/1985 discloses a system in which, when a slip of only one side of the left and right driving wheels is larger than that of the other side thereof, the one side thereof alone is braked and, when slips of both the left and right driving wheels are large, the both sides of the driving wheels are braked and further the output torque by the engine is caused to be reduced. The prior art systems as disclosed in the above patent applications are such that the braking of the driving wheels by the brake is primarily utilized and the reduction in the output torque of the engine is secondarily utilized.
Since the prior art systems are mainly the brake for the slip control for the driving wheels as described immediately above, they have various disadvantages that follow.
As the brake is continuously employed for the slip control, and is likely to be abused, it is disadvantageous in terms of durability.
As the brake is connected to the driving wheels to an extent greater than the engine, the use of the brake is advantageous in terms of a response to the slip control by the brake; however, the use of the brake is likely to cause a shock in the course of driving, leading to rendering the driving feeling poor. When the slip control is elaborately conducted mainly using the brake, consideration should be given to the driving feeling, whereby a braking apparatus should be rendered more complex and larger than conventional ones.
The use of the brake causes the engine to output a surplus of the torque equivalent to the braking force so that it is disadvantageous in terms of an effective use of energy leading to fuel saving.